Is the card you bought compatible with your camera? Older cameras cannot work with newer versions of memory cards. SD, SDHC,SDXC etc. have different form factors and old model cameras are not compatible with newer versions.
If the problem is related to the card, you might like to re-format the card from a computer before inserting it in the camera and then format it again from the camera.
Check and verify that before you try other solutions.
Is this an SD card or SDHC card ? If so, have you inadvertently set the write-protect switch to 'ON' ?
To read it on your computer, you will need to get a USB card reader that will read your particular card, then connect this to your computer, which should treat the card as an extra drive.
You say the card has been in use for several years - it may just be corrupted or at the end of its useful life - I should consider replacing it soon.